Structure and Conformation of Polythiophenes

The solution studies indicate that the ordered supramolecular disk structures with globally ordered extended conformations of polythiophene are desirable precursors to self-assembled PAT films, whereas the needle-like structures do not pack well and cannot give a globally ordered structure. This discovery implicitly has a profound effect on film casting and the resultant solid-state order (discussed below). The light-scattering data also suggest that solution spectra will vary with temperature, concentration, solvent composition, and the thermal history of the sample. 

Having these situations as a background, we describe in this chapter the electronic properties of the polythiophenes. Special attention is directed to (i) the structure and conformation (section 2.2) and (ii) various electronic processes (section 2.3) of these materials. The latter section mainly deals with the electronic properties relevant to the charged states. 

We have studied the electronic properties of the polythiophenes in relation to their structural and conformational characteristics. Their electronic properties come out of the TT-conjugated system extended along the polythiophene backbone. Since the electronic state is strongly coupled to the backbone conformation, a variety of electronic properties show themselves depending on the conformations of polymer chains. 

Nilsson et al. designed an experiment to test the ability of a polymer to directly detect conformational changes within peptide/protein structure [27]. Using the zwitterionic polythiophene derivative, POWT, they succeeded in detecting the distinct conformations of synthetic peptides. The mechanism was concluded to be based on the polymer side chains charged interactions and hydrogen bonding with the proteins. 

Two Hell UPS spectra of poly(3-hexylthiophene), or P3HT, compared with the DOVS derived from VEH band structure calculations [83], are shown in Figure 5-14. The general chemical structure of poly(3-alkylthiophene) is sketched in Figure 5-4. The two UPS spectra, were recorded at two different temperatures, -h190°C and -60 °C, respectively, and the DOVS was derived from VEH calculations on a planar conformation of P3HT. Compared to unsubstituted polythiophene, the main influence in the UPS spectra due to the presence of the hexyl... 

The coupling of the electronic structure of the conjugated polythiophene to the conformation can account for chromic phenomena as well as for aspects of electronic transport of the materials in their undoped or doped states. The order-disorder transition of the polymer chains, between a planar and a non-planar state, is governed by molecular mechanisms related to the nature and regularity of side-chain substituents. 

It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained firom substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regioregular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and polyisocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. 

It is also important in structural studies of the polythiophenes and oligothiophenes to see how completely (or incompletely) the S-trans conformation is retained. For this purpose, crystallographic structures of various oligothiophenes are presented. We stress the effects of the side- and/or end-groups attached to the thiophene backbone on the crystal structure and molecular conformation. Another important issue is associated with the structure of the conducting (doped) form of the materials. If its structure is determined in... 

For polythiophene, Mo et al. [102], Bruckner and Porzio [135], and Yamamoto et al. [84] proposed a similar structure with lattice parameters around a = 7.8 A and 6 = 5.6 A (for both the orthorhombic and monoclinic lattices with negligible difference), a and b and their ratio (approximately 1.4) resemble those listed in Table 8.5, and so the presence of the herringbone structure is assumed, c (7.8 A) is consistent with the size of two repeated thiophene units, or the distance of C5-C4 (7.77 A) in DMQtT (see Figure 8.27). Using the Rietveld method [136], Bruckner and Porzio [135] have confirmed the chain planarity, or the fully-stretched S-trans zigzag conformation of the polythiophene backbone. 

Additional measurements are ne ed in order to assess whether the n electrons have a major influence upon the local correlations or not The first ones were obtained on poly-n-alkylthiophenes, where it was found that the statistical length of an isolated poly-Sbutylthi hene chain is equal to 45 A (see chapter 6). Obviously the conjugated backbone of polythiophene is markedly different ftom polydiacetylene or polyacetylene, so a direct comparison between the different statistical conformations may not be meaningful. Nevertheless, it remains that the observed flexible behavior of the poly-3butylthiophene is a first and direct evidence that a conjugated backbone structure does not necessarily mean a stiff chain behavior. 

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